"Certainly there are still a lot of information that is classified about that event but we have enough evidence to know how it came about," Mehrzad Boroujerdi, professor and chair of the political science department at Syracuse University in New York, said.
There have been numerous investigations into 9/11 terror attacks and video evidence of who the terrorists who carried out the attacks were, Margaret Gilmore of London-based security think tank the Royal United Services Institute reminded.
"There is no reason to believe any of the conspiracy theories which are not based on firm evidence. The account that is widely accepted and backed by evidence is the most accurate count of what happened on 9/11 and the run up to it," she said.
The governments can be blamed to some extent for the 9/11 tragedy though as Western military operations in Middle East destabilized situation there thus giving a boost to rise in terrorism, experts noted.
"Politics does not like vacuums and it is natural for some actors to try to fill the void. Hence, it is therefore logical to argue that the war on terror contributed, albeit inadvertently, to the rise of terror attacks," Boroujerdi said.
"It is safe to say that in 2016 the world is not any safer than it was in 2001," Boroujerdi said.
According to Yoram Schweitzer, head of the terrorism program at the Tel Aviv-based Institute for National Security Studies, the post-9/11 society faced the phenomenon of "grand terror in a true sense of the meaning."
"It's terror from the Islamic State, aimed at population which is being conquered, and at the minorities. Plus, there is also new type of terrorism in the sense of the weapons. Moreover, social media became so vocal that we live in an atmosphere of a grand terror as never before," the expert said.
On September 11, 2001, al-Qaeda suicide attackers hijacked four passenger planes in the United States, crashing two into New York City's World Trade Center towers, another into the Pentagon and the fourth was sent in the direction of Washington DC, presumably to attack the White House or the Capitol. Some 3,000 people lost their lives in the terrorist attacks.
